Composite resin particles being of the so-called semi-IPN structure, each of which comprises a crosslinked polymer core and a number of linear, non-crosslinked, thermoplastic polymer chains intertwined with said core through partial or thorough penetration, can exhibit both film-forming property of the thermoplastic polymer chain portion and polymer filler effect of the crosslinked polymer core portion and can formulate various polymer blends with other coating use resins which are compatible with said thermoplastic polymer. Therefore, great hopes are entertained of such resin particles either singularly or in the combination form with other resins as additives in various technical fields including paint, polymer filler, optical material, adhesive, dental material, medicine delay releasing material and the like.
However, such composite resin particles of the semi-IPN structure heretofore proposed have been prepared by the double-stage emulsion polymerization (the so-called core-shell process) (e.g. L. H. Sperling; Interpenetrating Polymer Networks and Related Materials, pages 75 to 78, 1981) and therefore, both of the crosslinked polymer core and thermoplastic polymer chains are inevitably limited to the radically polymerizable resins only. It is, therefore, a principal object of the invention to establish a novel process for the preparation of composite resin particles of semi-IPN structure, each particle having a crosslinked polymer core and a number of linear, non-crosslinked, thermoplastic polymer chains intertwined with said core, in which both of the core polymer and chain polymer are freely selected from various resins including vinyl resin to be prepared by a radical polymerization technique and other resins to be prepared by non-radical polymerization as polyester, polyether, polyurethane, polyamide, epoxy resins and the like. Another object of the invention is to provide novel composite resin particles having improved toughness, softness, transparency and the like, prepared by the present process.